What does the color of the feathers have to do with the distance of migration? | Image source: pexels.com
Editor's Note
What does the color of the feathers have to do with the distance of migration? Scientists in Germany and New Zealand have found that migratory birds have lighter feathers than resident birds.
Written by | Wu Lei
Editor-in-charge | Chen Xiaoxue
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The colorful plumage is one of the breathtaking features of birds. In addition to reflecting the unique "aesthetic" preferences of different species, the bright plumage is also closely related to the ecological habits and living environment of birds. Since dark colors absorb heat more easily, biological colors in colder regions should be darker without taking into account other factors – this is known as Bogert's law [1]. Although this theory is based on variable temperature animals, numerous studies have shown that ambient temperature and heat trade-offs also affect the color of birds' body surfaces [2-4]. In other words, plumage also plays an important role in the heat regulation of birds.
This year, two studies have invariably pointed out that migratory individuals fly higher during the day than at night [5, 6], perhaps in order to offset the effects of solar radiation during the day with higher altitudes (lower temperatures). This further hints that heat regulation is an unavoidable challenge for migratory birds.
So does the shade of the feather also play a role in heat regulation during migration? In 2015, a study of 489 hermaphrodite species was used for analysis and found that lighter plumage was associated with migratory behavior, but the study also mentioned a series of ecological and behavioral factors such as reproduction in subtropical regions and male participation in brooding, and did not exclude the influence of disturbances such as climate [7]. Therefore, whether the plumage is related to migration remains an open question.
In an article published in Contemporary Biology on December 6, 2021, researchers at the Max Planck Institute for Ornithology in Germany and Massey University in New Zealand based on the Handbook of the Birds of the world) (birdsoftheworld.org) of images that study the correlation between plumage and migratory behavior of 10,618 bird species (22,325 images in total) [8].
The authors divide birds into residents, short-distance migrants (<2000 km), variable migrants (only some populations migrate >2000 km), and long-distance migrants (>2000 km). After controlling for factors such as climate, habitat type, body size, and sex, they found that the plumage of the resident birds was darker than that of short-distance migratory birds (female p<0.001, male p=0.014), and significantly deeper than long-distance migratory birds (female p=0.004, male p=0.0036). (Figure 1)
Fig. 1 Correlation between migration distance and feather shade | Image source[8]
From resident birds to long-distance migratory birds, the distance of migration gradually increases, and the plumage also shows a gradually lighter pattern. The above conclusions are based on the demarcation of the migration distance according to the boundary of 2000 kilometers, that is, the migration distance is regarded as a categorical variable. So will removing this layer of artificial processing lead to a consistent conclusion? The authors statistically found that this trend persists even when the migration distance is treated as a continuous variable (using the original migration distance value).
In birds, different taxa have different rates of speciation, so will there be some special taxa (such as fast-diverging, high-numbered taxa) that have a greater impact on the statistical results?
In order to eliminate this effect, the authors divided the birds according to their orders (the classification level is: boundary, phylum, order, family, genus, species) and analyzed them again. (Figure 2) Statistics show that the plumage brightness of birds in 12 orders (including passeriformes, orioles, geese, pigeons, etc.) show a positive correlation with migration distance, of which 7 orders have statistical significance, which is consistent with the above results; 3 orders show a negative correlation relationship, that is, the longer the migration distance, the darker the feather color, but it is not statistically significant.
Fig. 2 Correlation between feather color and migration distance of birds of 15 orders | Image source[8]
There is a strong link between plumage and migration, so why do some species not conform to this trend? The authors argue that the plumage of birds is constrained by many factors, and it is difficult to fully explain them with a single factor. For example, there is a hypothesis that the back of migratory birds is darker in color because the back of the bird with higher temperatures reduces friction and thus increases the efficiency of flight [9].
Fig. 3 Direction of airflow above the wings | Image source[9]
Light colors can reduce heat absorption and reduce the effects of strong solar radiation during the day; dark colors can increase surface temperature, reduce air friction, and thus reduce physical energy consumption during migration. How should birds weigh these two contradictory conditions?
The authors speculate that if the effect of temperature on friction is taken into account, the darker migratory birds may have a greater difference in day-night migration height and prefer to migrate at night—two traits that combine better plumage-behavior to further optimize energy use efficiency during migration. That is, the final evolutionary result is a trade-off under the influence of various constraints.
"The results of this article suggest that there may indeed be a general pattern of lighter plumage in migratory birds, which is related to temperature regulation during migration." Song Gang, an associate researcher at the Institute of Zoology of the Chinese Academy of Sciences, commented.
He said the brilliance of this study is based on the important clues found by previous research, combined with the core connotation of the basic principles in ecology, that is, body temperature regulation, "through the digital measurement of the color of all bird atlases and more accurate statistical analysis, so as to find the general ecological adaptation laws hidden behind the colorful feather color changes." ”
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